In order to be able to correctly assess the effect of the brightener in paper for different spectral distributions of the target observation illumination, measuring methods are currently known for example from the documents EP 1 775 565 B1 and European patent application No. 07 110 191.9 of Jun. 13, 2007 (corresponding to U.S. patent application Ser. No. 12/136,373 of Jun. 10, 2008) in which two measurements are taken using different illumination spectra, and the resulting measurement values are evaluated on the basis of a mathematical or physical model. The measuring methods described in these documents are based on a measurement using a white illumination and a subsequent measurement using an exclusively UV illumination.
Color measurement charts which are usually referred to as test charts are used for color management applications. Such color measurement charts or test charts normally include a two-dimensional arrangement of color measurement fields; in the simplest case, however, a test chart can also consist of a single row of color measurement fields, wherein in most cases, this is then referred to as a color measurement strip. Various hand-held color measurement devices for gauging such test charts (in rows and/or lines) during scanning operations are known, for example the devices “i1” and “ColorMunki” of X-Rite Europe GmbH, Regensdorf, Switzerland. Other hand-held color measurement devices for scanning operations are described in all their details in the documents WO 2006/117598 A1, U.S. Pat. No. 7,345,763 B2 and U.S. Pat. No. 6,590,648 B1.
When gauging test charts during scanning operations, for example by means of the cited known devices “i1” or “ColorMunki”, the hand-held color measurement device is preferably guided over a respective row of the color measurement fields on the test chart by means of a mechanical guide, wherein a multitude of individual measurements are automatically taken consecutively, at short intervals. The position of the color measurement fields on the test chart is ascertained from the spectral measurement data by evaluating the differences between the measurement values of the consecutive measurements. A detailed explanation is for example to be found in the document U.S. Pat. No. 6,590,648 B1. This method requires that measurement values between adjacent color fields do not fall below a minimum color difference. Furthermore, color measurement devices are also known, for example from the document U.S. Pat. No. 7,345,763 B2, which are already suitable for scanning operations and comprise an integrated distance measurement sensor which determines the position of the color measurement device during the scanning process. The knowledge of the position of the device during each individual measurement can then be adduced as an aid to identifying the measurement fields.
If an exclusively UV illumination is used in a measuring process, a sufficient difference in the reflection of the adjacent color fields in the UV range is required in order to determine the position of the measurement fields from the measurement data. However, this means a massive restriction in the configuration of the test chart.